Electrochemical sensors for analyzing gases have existed for quite some time. Such sensors include the Clark Cell described in U.S. Pat. No. 2,913,386 issued Nov. 17, 1959, and the Ross-type Cell described in U.S. Pat. No. 3,260,656 issued on July 12, 1966 to James W. Ross, Jr. These apparatus employ liquid electrolytes. The use of liquid electrolytes necessitates a chamber or holding means for confining the liquid electrolyte in the electrochemical sensor. As a result, these electrochemical sensors are often large and bulky and suffer from other disadvantages that can include slow response times and variations in sensor readings due to stresses applied to the electrodes caused by temperature and/or pressure variations.
An attempt at solving some of these problems is described in U.S. Pat. No. 4,076,596 issued to Connery et al. on Feb. 28, 1978. This patent discloses an electrolytic cell for measuring the concentration of a species in a fluid material. The electrolytic cell is constructed by depositing closely spaced, interleaved inert electrode surfaces on the surface of an insulating substrate and covering the electrode surfaces with a thin film of solid electrolyte and a permeable membrane. The Connery et al. apparatus is a completely self-contained electrochemical sensor which requires no liquid electrolytic solution to function. This type of sensor is quite desirable because it minimizes the problems associated with slow response times and bulky sensors.
However, several problems were discovered during the fabrication of the Connery et al. type apparatus. For instance, the electrolyte coating had a tendency to peel off of the substrate. Moreover, application of electrolyte coatings over electrodes as in Connery et al. often results in holes, dimples or warps in the structure of the electrolyte coating at the edges of the electrodes. These structural defects in the electrolyte film are probably due to poor step coverage at the electrode edge by the electrolyte coating and these defects could cause short circuits or other defects in the structure that cause poor operating specifications. The electrolyte coatings also exhibit a tendency to tear near the edges of the electrodes during the coating processes. This tearing adversely affects the properties of the sensor. Another problem that is typically encountered as a result of coating a solid electrolyte layer over electrodes is that stresses due to temperature, humidity and/or pressure often distort the electrolyte layer and thereby adversely affect the properties of the sensor. Finally, coating the electrolyte layer over the uneven surface of the electrodes often results in inconsistent electrolyte coating thicknesses and hence, inconsistent sensors.
Accordingly, it is the primary object of the present invention to provide a process for spin coating electrolytes which overcomes the disadvantages of the prior art.
It is a further object of the present invention to provide a process for spin coating a solid electrolyte layer onto the surface of a substrate to obtain thin, smooth and consistent electrolyte coatings under which electrodes may be located.
It is a still further object of the present invention to provide methods of coating a solid electrolyte layer on the surface of previously deposited electrodes which results in smooth and consistent coatings of electrolyte.
It is a still further object of the present invention to provide a method for making planar electrochemical sensors which are completely self-contained and include a solid electrolyte.
It is a still further object of the present invention to provide a method for making sandwich-type electrochemical sensors which incorporate several layers of solid electrolytic material.
It is a still further object of the present invention to provide methods of making electrochemical sensors having increased resistance to stresses caused by temperature, humidity and pressure.
These and other objects of the present invention will be apparent to one of ordinary skill in the art from the summary of detailed description which follow.